摘要:

ABSTRACTThe complex pattern of biological accretion, internal sedimentation, early lithification, and biological destruction, that characterizes modern reefs and many fossil reefs has been recognized in archaeocyathid‐rich patch reefs of Lower Cambrian age in the Forteau Formation, southern Labrador.Patch reefs occur as isolated masses or complex associations of many discrete masses of archaeocyathid‐rich limestone and skeletal lime sands, surrounded by well‐bedded skeletal limestones and shales. Each reef is composed of many loafshaped mounds stacked on top of one another. The limestone of each mound comprises archaeocyathids andRenalcis or Renalcis‐like structures in a matrix of argillaceous lime mud rich in sponge spicules, trilobite and salterellid skeletons. Numerous growth cavities roofed by pendantRenalcis‐like organisms andRenalcisare partially to completely filled with geopetal sediment indicating that much of the matrix was deposited as internal sediment.Two stages of diagenetic alteration are recognized: (1) syn‐depositional, which affected only the reefs, and (2) post‐depositional, which affected both reefs and inter‐reef sediments. On the sea floor reef sediments were pervasively cemented and fibrous carbonate was precipitated in intraskeletal and growth cavities. These limestones and cements as well as archaeocyathid skeletons, were subsequently bored by endolithic organisms. Later post‐depositional subaerial diagenesis resulted first in dissolution of certain skeletons and precipitation of calcite cement above the water table, followed by extensive precipitation of pore‐filling calcite below the water table.These carbonate reefs are similar in structure to the basal pioneer accumulations of much younger lower and middle Palaeozoic reefs. They did not develop into massive ‘ecologic’ reefs because archaeocyathids never developed the necessary large, massive, hemispherical skeletons.This occurrence indicates that reefs developed more or less coincident with, and not long after, the appearance of skeletal metazoan

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00299.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTKarlskaret fan, with a radius of less than 11/2 km and dominated by debris‐flow conglomerates, is one of numerous alluvial fans built out from the fault margins of Hornelen Basin (Devonian, Norway). The fan body is more than 170 m thick proximally, consists of four main coarsening‐upwards segments and thins distally by a rising of its base and by a vigorous interfingering with very fine‐grained sediments originating from an adjacent, impinging floodbasin system.Within the entire fan body, and within individual lobes, is a proximal‐distal (and vertical) facies change from sheet‐like, polymodal debris‐flow conglomerates through matrix‐rich conglomerates that are commonly distorted by loading, slumping and faulting, to remarkably sheet‐like, matrix‐rich granule sandstone of subaqueous debris‐flow origin.Because the alluvial fan prograded into an actively aggrading floodbasin the primary fanglomerates, themselves having been subject to some sorting on the fan surface, incorporated large quantitites of very fine sediments. This inclusion of fines, effectively a textural inversion on the lower fan reaches, frequently led to remobilization and resedimentation of material beyond the fan toe. Anomalous maximum particle size/bed thickness relationships and a variety of graded textures within these resedimented beds suggest deposition in lacustrine areas of the

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00300.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTThe Upper Silurian Keyser Limestone is a relatively thin (<85 m) unit of lagoonal, barrier, and shallow offshore sediments that crops out in the central Appalachians. Lithologies include massive micritic limestones to calcarenites, calcisiltites, and calcareous quartz arenites. The barrier lithofacies is preserved predominantly as tidal inlet channel‐fill. Its presence is supported by two lines of evidence: (1) the sequence of sedimentary textures and structures resembles that observed in modern inlets, and (2) the sequence occupies a position immediately above a disconformity, and is accompanied by an abrupt vertical change in faunal diversity, which is interpreted as representing the transgression of open marine over back‐barrier environmentsThe inlet channel sequence comprises fine‐ to medium‐grained, well‐sorted quartz arenites that disconformably overlie sediments deposited on carbonate tidal flats (laminated, mudcracked pelmicrites). The sandstone displays a fining‐upward texture, and contains a broken and abraded mixed fauna. Cross‐bedding is bipolar, with major modes oriented obliquely to depositional strike. Decimetre‐scale sets of planar and trough cross‐beds grade upward to centimetre‐scale sets of ripple cross‐lamination, washed‐out ripples, and plane beds. This sequence represents the change from deep to shallow channel environments, and is attributed to lateral inlet migration. The inlet sequence was preferentially preserved during marine transgression because of its relative thickness and lower stratigraphic position with respect to overlying and adjacent barrier‐beach sediments.The vertical relationships of this inlet‐lagoon complex emphasize that care must be taken in interpreting shallow‐water transgressive sequences. Vertical ‘jumps’ in faunal diversity accompanied by scour surfaces could be misconstrued as major unconformities. Instead, such sequences may represent the shoreface erosion normally associated with the transgressive migration of barrier islands. Whether or not the faunal jump is accompanied by a barrier lithosome is greatly dependent on the geometry, frequency, a

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00301.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTExperiments have been conducted in a 10 m long laboratory flume to investigate the bedforms which develop from fine, cohesionless sediment beds. Two grades of near uniformly sized silica grains (of median nominal diameters 15 and 66 μm) and six grades of micaceous flakes (ranging in median nominal diameter from 15.5 to 76 μm) were used. A steady subcritical water discharge, which was increased in steps after several hours, was applied to a flat bed of each grade. The developing bedform sequence for fine granular beds was identified as many small‐sized primary ripples, isolated primary, transverse primary, secondary ripples and then possibly dunes; this development was almost the same as that observed for coarser grains. The sequence for fine flake beds differed from grains. Only the single bedform type of parting lineations was observed; with increased discharge, the lineations began to oscillate and eventually enter into fluid suspension. The low discharge parallel lineations were thought to be generated by ‘streaks’ or lanes of transversely alternate high and low velocity fluid which have been reported to exist in the viscous sub‐layer of a turbulent‐smooth boundary, whilst the higher discharge wandering lineations were attributed to low velocity stre

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00302.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTOxygen isotopic compositions of chert and calcite cements in the Lake Valley Formation indicate that these diagenetic features cannot be equilibrium co‐precipitates in spite of their coexistence in the same interstices. Petrography of megaquartz and non‐ferroan calcite cements indicates that both are original precipitates that formed during pre‐Pennsylvanian time at shallow burial depths (<215m) implying precipitation temperatures less than 30°C. Under these constraints the δ18Os of megaquartz (mean =+27.00/00SMOW; range =+ 24.8 to + 28.90/00) and calcite (mean =+ 28.00/00SMOW; range =+ 27.3 to + 28.40/00) are best interpreted as unaltered since precipitation; thus, they must reflect the oxygen isotopic composition of pre‐Pennsylvanian pore waters. Microquartz and chalcedony are interpreted to have formed from recrystallization of pre‐Pennsylvanian opal‐CT precursors, and therefore probably re‐equilibrated during recrystallization in late or post‐Mississippian time.We propose a model integrating the isotopic data with regional petrographic and sedimentological data that explains the greater consistency and generally greater δ18Os values of the calcites compared to those of the cherts. This model is one of chertification and calcite cementation in a regional meteoric phreatic ground‐water system, the seaward terminus of which moved southward during lowering of pre‐Pennsylvanian sea level. The calcite cements and some of the opal‐CT precursor to microquartz and chalcedony are interpreted to have formed in the more seaward portions of the groundwater system. The megaquartz precipitated in the more inland parts of the phreatic groundwater system where rainfall was isotopically lighter and more variable. As such, the δ18Os of the megaquartz reflect the isotopic composition of groundwaters in areas undersaturate

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00303.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTAbout 4100 samples of suspended matter were collected by filtration of surface ocean waters in three large regions on the western sides of oceans and two on the eastern sides. Comparison of results shows that the non‐combustible fraction (chiefly detrital clays and silts with some siliceous and calcareous skeletal debris) generally dominates along the western sides of oceans, where large contributions of solid detrital sediment are made by rivers that drain much of the adjacent continents. The combustible fraction also is important off these rivers, but it is more important (both relative to the non‐combustible fraction and in absolute terms) along the eastern sides of oceans, where upwelling is inte

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00304.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTFor a small sandur in the Canadian Arctic, trend surface analysis of fifty‐three samples shows that less than 40% of clast size variation is explained by downsandur fining. Further variations are explained by considering different levels of the sandur surface separately. In the main flow area, which carries water even during periods of low flow, channel deposits are invariably coarser than bar deposits although the difference lessens and overall sorting improves with distance downsandur. The flood surface area, which carries water only during high discharges, is less affected by downsandur fining than the rest of the sandur, and the bar‐channel relationship is weaker. The high flood surface which occupies most of the proximal zone of the sandur is probably an abandoned main flow surface, as indicated by clast size characteristics similar to those on the present main flow surf

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00305.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

Book Reviewed in this article:Sedimentary Rocks, by F. J. Pettijohn.Sedimentary Petrology. Part 11. Sediments and Sedimentary Rocks 1(Second revised edition), by Hans Fuchtbauer. E.Sediment Petrologie, Teil II. Sedimente und Sedimentgesteine (3. Auflage), von Hans Fü;chtbauer und German Müller. E.Origin of Sedimentary Rocks, by Harvey Bla

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00306.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY